Production of extremely thin filaments



May 10, 1949. F. ,e. BROCKMAN ETAL 2,469,947

PRODUCTION OF EXTREMELY THIN FILAMENTS Filed April 13, 1944 N I mm v m my E 0 M W? m IBW 6w A mw Patented May 10, 1949 PRODUCTION OF EXTREMELYTHIN FILAMENTS Frank G. Brockman, Woodbury, and John W. Wescott, 2nd,Chatsworth, N. J., assignors to Socony-Vacuum Oil Company, Incorporated,a corporation of New York Application April 13, 1944, Serial No. 530,950

1 Claim. 1

This invention relates generally to a method of producing ribbon-likefilaments and more particularly to a method of producing extremely finefilaments. A specific application for such filaments can be of use inbolometers, thermocouples, or other measuring instruments employed whereit is desired to detect and/or measure extremely small amounts ofradiant energy.

In the prior art workers have prepared similar filaments by usingWollaston wire and removing the silver from the thin platinum core bythe action of nitric acid; or by welding platinum foil to heavier silverfoil and rolling this double foil to suitable thickness, cutting outribbons and then dissolving away the silver with nitric acid; or byelectroplating nickel upon stainless steel, mechanically removing thethin nickel plate and cutting the resultant thin foil into filaments.The first two methods present numerous difficulties, not the least ofwhich is the effect of gas evolution upon the filament during the acidtreatment. The last method is obviously very difficult when the foil isextremely thin.

The method of the instant invention is free of the difficulties inherentin those of the prior art enumerated above; is based on soundelectrochemical principles; and produces extremely thin fifilaments withconsiderably greater ease than any of the previous methods.

Broadly stated the method of the present invention comprises plating themetal selected for the filaments on a sheet of base metal, cutting thecomposite metal sheet into ribbons, securing each ribbon to a frame toform a voltaic couple, and immersing the frames and ribbons carriedrespectively by each frame into an aqueous bath of a cyanide of analkali metal whereby the base metal is electrolytically dissolved.

Therefore, the primary object of the present invention resides in theprovision of a method of producing extremely thin filaments havinguniform physical, thermal and electrical characteristics.

Another object of this invention resides in the production of filamentshaving a thickness of approximately 0.1 micron (approx. 5

inch) which is uniform throughout.

Still another object of this invention resides in the provision of amethod of electrolytically producing extremely thin metal filamentswithout the evolution of gases in the vicinity of the filament.

This invention further contemplates the use of a voltaic couple inelectrolytically dissolving the base metal from the filament metal.

Still another object of the present invention resides in the provisionof a method of production of extremely thin filaments whose thicknesscan be duplicated with accuracy.

This invention further contemplates a method of producing extremely thinfilaments that can be practiced economically and with facility.

Other objects and advantages of the present invention will becomeapparent from the following detailed description when considered withthe drawings, in which Figure l is a longitudinal view of the frame andribbons shown in part in vertical section; and

Figure 2 is another 1ongitudina1 view of the frame and ribbons differingfrom that of Figure: 1 in that the vertical section is taken at rightangles to that shown in Figure 1.

Figure 3 is a view of the frame and ribbons in the bath.

According to electrochemistry, if a voltaic: couple formed by two metalsthat differ widely in the electrochemical series, is immersed in an.aqueous solution of a cyanide of an alkali metal, the metal of thecouple having the more positive: position in the electrochemical serieswill be dissolved. The chemical reaction that takes place is that the+ions of the metal having the more positive position in theelectrochemical ,series will enter the aqueous solution and will reactwith the negative cyanide ions to form a complex with the cyanide. Theositive ions of the alkali metal used migrate to the metal of the couplehaving the most negative chemical potential where they pick up negativecharges and react with water forming stable hydrogen.

The stable hydrogen is evolved from that electrode in the form of gas.This reaction will continue until the metal of the couple having themost positive position in the electrochemical series has been completelydissolved.

Reaction at the copper electrode:

It will be understood that the metals selected for the voltaic coupleshould be separated by a fairly large potential diiferen'ce in thevoltaic 3 order of metals in aqueous solutions of a cyanide of an alkalimetal.

Utilizing the above theory, a base metal, such as copper, is selected.This metal should be in the form of, or reduced to, a sheet of foilhaving 5 a moderate thickness; for. examplee0.0015: inch- Then the metalfrom'which the thin-filaments" are to be formed is selected, forexample, nickel. The procedure then consists in plating the copper foilwith the nickel to a thickness.-:equal -tosthat 1.10;, desired in thefinal filament by using any of thee. well-known electroplating methods.Although it is possible to deposit the nickel on ;the--copp er-* foil byother methods such as evaporation or" sputtering, the plating process isthe-prefer ableone because of the ease with which a coating of thedesired thickness can be appliedz.

After the nickel is deposited on the copper to. the desired thicknessribbons of the required width and length are cut. These ribbons aremounted ina frame or 'frames, as shown in Figures 1 and 2, by anyappropriate means, such as soldering. Thisframe servesas a supportforthe final filament orfilaments;

Referring to the drawings; particularly Figures 1 and 2, thecopper-nickel ribbon M] ismountedi in the platinum frameby solderingthe-nickel side ofthe ribbon at each end-tothe-ends-= of platinum wiresHand-I23. Wires-1I and l2 areheld in spaced relationship by" glass beadsI3' and M. A third platinum wire l5 isalsoheld in parallel spacedrelationship'by-the glass beads l3 and M. This wire is electricallyconnectedto the center of the nickel side of the ribbon ID by solderingthe nickel to the platinum bridge I6. The free ends of all three wires'll; IZ and- I5= extend through openings in the glassplu-g [1. Each wireis'sealedin'its'openingl As shown in Figure-3 the: assembled ribbon andplatinum frame is submerged in a bath!!! formed of a cyanide-ofan alkalimeta-1'.

Obviously the platinum frame can be-formed inother configurationsw Theone disclosed is-- that used in the manufacture of bolome-ter ribbonsand which wascarriedover into a finished instrument where it'servedas-apermanent sup' port" for the ribbons and meansfor making elemtricalcircuit connections to them.

The-copper foil isnow dissolved, leaving thethin layer of nickel whichwas originally Kie posited on the-copper'foil. Thisis accomplished byimmersing the ribbon" while 'm-ounted in-its-=- frame in anaqueous-solution ofacyanideof an alkali metal 1 such as potassium orsodium; and" making a metallic connection between therib hon, byway-ofthe'framegandan-electrode of? platinum, which is immersed in thesame solu tion, to form a voltaic couple; The-concentration of theaqueoussolution of a cyanide-of an-alkali metal is not critical butaiTectsthe'" time re-- quired to complete the process." It 'is'obvious-- that a, mere trace of-a cyanide ofan alkali-*metal wouldunduly prolong. the process. If the frame is constructed of platinum,the electrode is superfluous. The couple-.formedbywhe' platinum and thecopper acts electrolytically, the copper .being dissolved away withoutthe-evolution of gas, which is so destructive to the-nickel filament.C'opious gassing occurs at the platinum, but this is notobjectionablezsincethe gas; does: not come into contact with thethinfilament, of nickel. When all of the copper is "dissolvedjgassing atthe platinum electrodeceases and, the extremely thin filament. ofnickeluin litscmountinmf-rame ism: obtained.

4 In practicing the above-described method, a number of extremely thinfilaments have been prepared. A typical example of these is a pair ofnickel filaments that were simultaneously produced each of which had thefollowing properties:

Length, 1.4 cm.

Width, 0.038 cm.

Thickness, 0.1 micron inch).

(approx. /1000000 A foil-of base metal 'is firstplated with-one ofthe-metals of which the finishedthermocouple is'forrned. Theplating-should cover a little overone -ha-lf ofthe--foil.-- Next; thesecondmeta-l of which the finished thermocouple is formed-is platedoverthe other end'of the base metal foil; 1 This plating shouldslightly-overlap -'the-* firstplating. Now, if the two -metals,-plate'don th'e base-metal, are more negative than'thebase: metal; it ispossible to -etch-away---the'base metal in-the manner described above inconnection" withthe extremely fine ribbon like' filaments.

Weclaim:

In-a method of producing an extremely'thin nickelfilamentbvplatingnickel on'copper foils to a thickness of'0.1 micron;cuttinggtheiplated" foil to-form a ribbon having, the length and;width-of the desiredfinished-filament; and "dis,- solving 'the copper;foil from the nickel .filament" by submerg-ing" the composite ribbon, ina bath formed of" an aqueous solutionjofa cyanide'of an alkali metal,the improvement which "come prises securingthe ribbon toiaplatinum.frame.v in conducting relationship therewith before ity issubmerged in the bathto form .avoltaic couple, v whereby-on submergingthe frame and, ribbon,

in a bath'the copper will be dissolved and}.

bubbling occasioned by liberation, of; hydrogen during dissolution of"the copper will'joccur, only, at'the platinum electrode 'Of thevoltaiojcouple.

FRANK G. BROCKMAN. JOHN VT. WESC'OTT,]2N,D.

REFERENCES CITED.

The following references; are., of,record-,in the:-

file. of: this: patent:

UNITED rSTATES, PATENTS Number Name Date 771,340 Van Housen et a1.Cote-4, 1904 8655688 Edi-sow; Sept. 10; 1907 909,8'3'1Stricker-Aufermann- Jan: 13'; 1909 1;709.,-8i01 Muller; Apr. 69 19-291,787,139 Cain" Dec: 30; 1939"- 2,052,962=. Booe Septl 1', 1936 OTHER.REFERENCES; I

Inorganic Chemistry by J. W. Mellor, vol. III; Longmans, .Green.&s Co,,New York, --1923,-- pages 502; 503..

A. Comprehensive .Treatiseon; Theoretical and Certificate of CorrectionMay 10, 1949.

Patent No. 2,469,947.

FRANK G. BROCKMAN ET AL. tified that errors appear in the printedspec'fication of the above numbered patent requiring correction asfollows:

Column 2, line 52, strike out that portion of the formula readingOu(GN)[+2Na =CuNa (CN) e Reaction at the platinum electrode: in line 46;

should be read with these corrections therein that the Patent Office.

It is hereby cer and insert the same abov and that the said LettersPatent the same may conform to the record of the case in Signed andsealed this 18th day of October, A.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

